There is a continuing expansion of the use of composite materials for a diverse array of applications. In particular, the use of composite materials in the aerospace field is continually expanding because of the strength-to-weight advantage provided by composite materials as opposed to metallic materials.
In using composite materials to manufacture articles, particular attention must be devoted to the design and implementation of the manufacturing process for the particular composite article to be manufactured. A primary consideration in the design and implementation of a manufacturing process for composite articles in the aerospace field is that such process must produce composite articles, on a repeatable basis, with minimal deviations with respect to the one or more of the design dimensions, i.e., thickness, width, length. Out-of-tolerance deviations with respect to one or more of the design dimensions may adversely affect the structural utility of the composite article, e.g., inability to integrate such an out-of-tolerance composite article in combination with adjacent components, but, even more importantly, out-of-tolerance deviations with respect to one or more design dimensions may adversely affect the design response characteristics of the composite article. This is a particularly important consideration with respect to the manufacture of composite articles for use in the aerospace field.
For example, the use of composite flexbeams in a helicopter main rotor assembly is becoming ever more commonplace. Representative examples of composite flexbeams are described in U.S. Pat. No. 5,431,538 (A Hybrid Composite Flexbeam for a Helicopter Bearingless Main Rotor Assembly) and U.S. Pat. No. 5,372,479 (A Flexbeam for a Helicopter Bearingless Main Rotor Assembly). A composite flexbeam for use in combination with a bearingless main rotor (BMR) assembly must have design dimensions that meet demanding tolerance specifications so that the design response characteristics of such composite flexbeams accommodate the bending strain, shear stress, buckling, and frequency conditions experienced during critical loading, i.e., flapwise, chordwise, torsional, and centrifugal loads, as a result of operation of the BMR assembly. The critical loading conditions include start up and shut down, which generate low-cycle, high-strain flapwise and chordwise loads, and forward flight conditions, which can generate high-cycle, high-strain loads such as 1 cycle/rev oscillatory flap and torsional displacements.
With respect to the criticality of the design dimensions of composite flexbeams, for example, a composite flexbeam must possess a certain minimum cross section to transmit the main rotor blade centrifugal loads into the rotor hub assembly. Conversely, however, the thickness of the composite material(s) comprising the composite flexbeam must be minimized to ensure that maximum allowable torsion shear strain limits are not exceeded. Flapwise and chordwise loads require additional material in the flexbeam to accommodate bending stresses. Such additional material, however, increases flexbeam stiffness, causing increased hinge offset. For a soft inplane rotor design, the chordwise flexbeam stiffness is governed by the need to establish the rotor chordwise frequency at about 0.7 cycle/rev. If the flexbeam is too compliant in chordwise flexibility, the BMR assembly is more susceptible to aeromechanical and structural instability. If the flexbeam is too stiff, however, chordwise loads will increase because of 1 cycle/rev resonance. The torsional stiffness of the pitch section should be minimized to keep pitch actuator requirements to a minimum. In contradistinction, however, the torsional stiffness of the pitch section must be sufficiently high to ensure buckling stability under edgewise loading.
Thus, it is evident that the design response characteristics of composite flexbeams are inextricably linked to, and dependent upon, the design dimensions of such composite flexbeams. Therefore, a process for manufacturing composite flexbeams must be designed to ensure that the finished composite flexbeams meet or exceed, on a repeatable basis, the demanding tolerance specifications delimited for the design dimensions of such composite flexbeams.